The present invention relates to a plastic injection molding machine and, more particularly, a plastic injection molding machine having a plasticizing unit and an injection unit.
Plastic injection molding machines typically employ plasticizing units and injection units for injecting resin into a mold. The plasticizing unit includes a heated cylinder which receives a plasticizing screw which is capable of rotational and axial movement inside the heated cylinder. Pellets of plastic are introduced into the heated cylinder and melted therein. Rotation of the plasticizing screw forces the pellets along the length of the heated cylinder until they form a pool of melted resin in front of the tip screw. A check valve provided on the screw tip prevents backflow of the melted resin as the screw is axially advanced into the cylinder by the injection unit thereby causing the resin to be forced out of the nozzle in the end of the heated cylinder. The nozzle on the end of the heated cylinder is sealed against the sprue bush of a mold and the action of forcing the resin into the mold is called plastic injection. Typically during the operation of the plastic injection molding machine the plasticizing screw in the cylinder, which both rotates and slides, develops high injection pressures typically in the area of 30,000 psi.
There have been various designs for the plasticizing and injection units of plastic injection molding machines in order to enable the plasticizing screw to both rotate and axially move within the heated plasticizing cylinder. A typical device is described and illustrated in U.S. Pat. No. 4,731,005 to Hehl. In the arrangement of the Hehl patent as best shown in FIG. 3, the injection piston 58 of the injection unit has the motor 63 attached directly to it for rotating the plasticizing screw. The motor in turn drives a quill 64 which is attached to the plasticizing screw 14. As the motor 63 rotates, the plasticizing screw 14 is driven via the quill 64 through a series of fixed couplings. As the pool of melted resin builds up in the front of the plasticizing screw 14, the screw is pushed toward the right along with the injection piston 58 which slides inside injection cylinder 57. Thrust bearing 55 permits this back force to the injection piston while permitting the screw 14 to rotate. When sufficient resin has been plasticized, motor 63 is stopped in order to stop rotation of the plasticizing screw 14 and high pressure fluid is delivered into the injection cylinder 57 and acts on the righthand side of the injection piston 58 which causes the injection piston 58, the quill 64 and the plasticizing screw 14 to be moved to the left so as to inject the plastic resin from the cylinder and into a mold. As can been seen from the foregoing discussion and particularly FIG. 3 of the Hehl patent, the motor 63 reciprocates back and forth with the screw and the injection piston 58. As a result of the fact that the motor is fixed to the injection piston and reciprocated therewith, the weight and inertia of the motor increases the wear on the bearings and seals of the assembly which is highly disadvantageous. In addition, a complicated attachment is required from the power supply to the reciprocating motor 63.
A similar arrangement is shown in U.S. Pat. No. 4,349,324 to Neff et al. where plasticizing screw 2 is rotated by motor 3 which in turn is mounted on injection piston 9 mounted inside injection cylinder 8.
FIG. 1 illustrates a different arrangement known in the prior art. As shown schematically in FIG. 1, the plasticizing screw 1 is directly mounted to a injection piston 2 which is provided with a bore having a spline which slides along a corresponding spline on quill 3. The quill 3 is rotated by motor 4 which is mounted on the stationary injection cylinder 5 and, therefore, does not reciprocate with the plasticizing screw 1. Upon rotation of the motor 4, the quill 3 is rotated and in turn rotatably drives injection piston 2 via the spline connection for imparting rotation to the plasticizing screw 1. A severe disadvantage of the arrangement of the prior art mechanism described in FIG. 1 is that the injection piston rotates at the same speed as the screw as well as reciprocating within injection cylinder 5 when injecting the melted plastic resin. Thus, the seals on the injection piston are subjected to very high wear from both the rotating and sliding action of the injection piston 2 within the injection cylinder 5.
A further known arrangement is shown in FIG. 2 of the instant application which is an arrangement employed by the assignee of the instant invention. In the design shown in FIG. 2, the plasticizing screw 1 is connected to quill 3. The quill 3 is in turn mounted within a quill insert by corresponding matching splines and the quill insert is connected to the output shaft of the motor 4. A large thrust bearing 6 and forward bearing 9 on the quill support the assembly inside the injection piston 2 which slides within injection cylinder 5. A sleeve 10 is provided around a portion of quill 3 and quill insert 8 and the injection piston 2 slides in sealing engagement on the sleeve 10. The sleeve 10 separates the hydraulic fluid used in driving the injection piston 2 from the lubricating oil used for lubricating corresponding splines 7 and bearings 6 and 9. Thus, the arrangement of this mechanism has the advantage of employing a stationary motor, large thrust bearings and having the thrust bearings exposed to lubricating fluids so as to improve the life thereof. The arrangement, however, does suffer from the disadvantage that the overall length of the assembly from the feeder tube 11 to the motor 4 is excessively long.
Naturally, it would be highly desirable to provide a plastic injection molding machine which overcomes the deficiencies noted above with regard to the prior art.
Accordingly, it is a principal object of the present invention to provide a plastic injection molding machine wherein excessive wear on the seals of the injection piston is avoided.
It is a further principal object of the present invention to provide a plastic injection molding machine as aforesaid wherein the thrust bearings which transmit force to the injection piston are continually lubricated with lubricating oil.
It is a still further object of the present invention to provide a plastic injection molding machine wherein the motor for rotating the plasticizing screw is fixed to the ejection unit housing.
It is a still further object of the present invention to provide a plastic injection molding machine as aforesaid which is not of excessive length.
Further objects and advantages of the present invention will appear hereinbelow.